Japanese Patent No. 941444 discloses a method for making water-soluble foundary patterns, wherein urea in powder for containing polyvinyl alcohol was melted to prepare a homogeneous mixture of urea and polyvinyl alcohol in the molten state, and the resultant molten mixture was formed into foundary patterns. In the method there were the following disadvantages: When the molten mixture was formed into patterns, the molten mixture has to be poured or injected into the patterns forming mould at temperatures below the melting temperature of urea. However, it required particular technique for the molten mixture to be formed into sound patterns, because a range of temperatures between the melting temperature and the solidifying temperature of the mixture was very narrow. Although a part of polyvinyl alcohol in the mixture was formed a solid solution with urea, the majority of polyvinyl alcohol was not melted. Thus, the molten mixture was mainly composed of urea, and had a narrow range of temperatures between the melting temperature and the solidifying temperature. As a result, the molten mixture was hardly formed into the patterns having a good surface quality. Furthermore, when urea had been melted before being added with polyvinyl alcohol, the molten urea was easily gasified, above the melting temperature (132.7.degree. C.). It was possible to add moisture into the molten mixture so as to enlarge a range of temperatures between the melting temperature and the solidifying temperature, but in this case there was raised another problem of weakening the strength of the resultant patterns. Accordingly, it was necessary for the proportion of the moisture to be limited within a range between 0.5% and 2.0% by weight, and as a result, it was impossible to obtain a wide range of temperatures between the melting temperatures and the solidifying temperature, which range has been required in the injection moulding process.